The detector of a chromatograph provides an analog signal corresponding to the concentrations of sample material flowing through it. Before the information is applied to an integrator, certain processing functions are generally carried out by a digital system. In most cases, the dynamic range of the signal is so great that the digital system would have to have an excessively large number of bits if a reasonable degree of resolution is to be attained, but only a reasonable number of bits are required if the signal is first translated into logarithmic form. This may be accomplished by applying the signal to an operational amplifier circuit in which the feedback is provided by a transistor. If the detector is a voltage source, such as a thermal conductivity detector, it is connected via a coupling resistor to the inverting input of the amplifier. The non-inverting input is connected to a point of reference potential, and the desired logarithmic signal appears at the output. It is essential that there be very little noise or other distortion at the output of the amplifier because any error will be multiplied many times when the antilog of the processed signal is taken.
To compensate for variations in the temperature of the junction of the feedback transistor, a second operational amplifier and feedback transistor are provided. If the feedback transistors are a matched pair, the changes in the output of the operational amplifiers due to temperature variations can be substantially cancelled by subtracting one output from the other.
As is well known, the current flowing through the feedback transistor associated with the first operational amplifier equals the input current applied to it. As the latter current approaches the maximum current for which the feedback transistor is designed, an error term in logarithmic operation due to the internal resistance of the device becomes significant. The error term appears as a linear component in the output of the first operational amplifier. When the value of the resistor coupling a TC detector to the inverting input of the operational amplifier is adjusted for the best compromise between current and voltage noise, it is found that the current can exceed the maximum operating current of the feedback transistor so as to introduce a linear component into the logarithmic output. It might seem at first that this could be eliminated by using feedback transistors having a sufficiently high maximum current rating, but the feedback transistors must be a matched pair, and the available matched transistors do not have high enough current ratings.